Erosive synovitis can be induced experimentally in rats by intraperitoneal injection of bacterial walls from group A streptococci. Genetic factors determine susceptibility to development of disease. High inbred rat strains develop marked mononuclear cell inflammation, most evident in peripheral joints. Low responder inbred strains show little evidence of pathology. This model may facilitate characterization of factors that potentiate the development of chronic destructive arthritis.